Food Preservation Processes

Food regulatory laws

In food process engineering, a governing law is the basic relation that describes how momentum, heat, mass or particle size changes in a unit operation, and examinations test whether each law is paired with the correct operation. Hagen-Poiseuille's law gives the laminar pressure drop of a Newtonian fluid in a circular pipe as \Delta P = 32\mu L V / D^2, so it belongs to fluid flow and pressure drop, not to molecular diffusion. At a fixed velocity, halving the pipe diameter therefore quadruples the pressure drop. Stefan-Boltzmann's law, q = \varepsilon\sigma T^4, gives the total radiant energy emitted by a black or grey surface and governs radiation heat transfer. Kirchhoff's law states that, for a body in thermal equilibrium, emissivity equals absorptivity, which is the basis of radiative heat exchange. Fourier's law, q = -k\,dT/dx, governs conduction, while Fick's law, J_A = -D_{AB}\,dC_A/dx, governs molecular diffusion. Stokes' law gives the drag on a sphere in creeping flow as F_d = 3\pi\mu d v, and balancing this drag with the net gravity force gives the terminal settling velocity v = (\rho_p - \rho_f) g d^2 / (18\mu). It is a momentum-transfer law and is not a mass-transfer law. Rittinger's law relates the energy of size reduction to the new surface area created, E \propto \Delta S, and it has no connection with vapour pressure. Raoult's law, p_i = x_i p_i^{\circ} for an ideal solution, relates partial vapour pressure to mole fraction and does not describe size reduction.

GRAS chemical preservatives

Generally Recognized As Safe (GRAS) substances are those whose safety for a specified use is accepted by qualified experts, and a GRAS preservative is valid only for its documented target organism and food matrix. A correct pairing therefore needs three elements to match: the preservative, the organism it inhibits, and the food in which that use is established. Sodium lactate is used in pre-cooked meat products to suppress bacteria such as Listeria and Clostridium botulinum. Sodium nitrite is used in meat curing preparations to inhibit Clostridium botulinum, and it is a classic example of a GRAS preservative matched to its organism and food. A pairing fails when the preservative is not used against the organism named, or when the stated organism is not the target of that preservative in that food.

Thermal processing safety vs quality

Heat processing has two competing outcomes. Microbial lethality, which represents food safety, rises with increasing process severity, meaning a higher temperature and a longer time, and it approaches a practical limit as the process is extended. Quality attributes such as colour, texture and vitamin content decline with the same severity. In a figure with these two trends, the rising curve that starts at the origin represents safety, and the falling straight line represents quality. Quality losses are generally described with a higher z-value than microbial inactivation, so a higher temperature for a shorter time preserves quality better than a lower temperature for a longer time at the same lethality. Yield is not the attribute depicted by either trend.

Cryogenic freezing refrigerants

In direct immersion freezing, the food is placed in direct contact with the refrigerant, so the refrigerant must be inert, non-toxic and not absorbed by the food. Liquid nitrogen boils at about -196\,^\circC and meets these requirements, which makes it the most commonly used refrigerant for direct immersion freezing. Halogenated refrigerants such as R-12 (dichlorodifluoromethane) and R-22 (monochlorodifluoromethane) are used in indirect vapour-compression systems, where the refrigerant does not contact the food. The name Freon is a trade name for a family of halocarbon refrigerants, not a specific refrigerant for immersion freezing.